Rapid expansion in use of portable electronic equipment such as mobile phones, notebook computers, camcorders, digital cameras and the like has led to increased demands for secondary batteries having a high-energy density as a power source for such equipment. In recent years, applicability of secondary batteries has been realized as power sources for electric vehicles (EVs) and hybrid electric vehicles (HEVs).
As examples of such secondary batteries, lithium secondary batteries comprising an anode of a carbonaceous material, a cathode of a lithium metal oxide, a separator of a polyolefin material and a non-aqueous lithium salt electrolyte are widely used. For optimal use in the electronic equipment of interest or vehicles, the lithium secondary batteries require excellent life characteristics. As such, efforts and attempts to improve a battery life are continuously undertaken, because the battery must undergo little decrease of the capacity even after repeated charge/discharge cycles.
Batteries undergo deterioration of life characteristics due to degradation of individual components caused by various factors. One of the main causes for the deterioration of the battery life characteristics is incorporation of impurities into the battery. For example, as the incorporation of water into the battery accelerates the degradation of the battery performance, Korean Patent Registration No. 414588 discloses a technique of inhibiting adverse side reactions and gas evolution by adsorption of water and water-borne by-products via the addition of zeolite to an electrolyte. In addition, Japanese Patent Application No. 2003-323916 A1 discloses a technique of suppressing battery degradation by adsorption and removal of hydrofluoric acid produced from the reaction between water and lithium salts, via the addition of zeolite to an electrode active material or the like.
However, according to the experiments conducted by the inventors of the present invention, it was confirmed that internal short-circuiting occurs to thereby sharply decrease the battery capacitance when metal impurities are incorporated into the battery, even after complete removal of water inside the battery or the by-products produced from the reaction of water with the lithium salts. Further, incorporation of large quantities of the metal impurities results in a failure to sufficiently fulfill functions of the battery. Therefore, maximum care should be taken to ensure that incorporation of the impurities does not occur upon fabrication of the lithium secondary battery.
However, since it is in fact impossible to completely block the incorporation of the metal impurities which may occur during fabrication processes of the electrode active materials, conductive materials and the like, there is a need for the development of a technique to ensure that the internal short-circuiting of the battery does not take place even upon incorporation of the impurities.